Canadian Customs is notorious for being ridiculous about paperwork and holding things at the border if they're not EXACTLY labelled in the way THEY want it.
A prime example that my FLGS/Army Surplus Store had was a Tent from the US that was laid up in Customs for months (Because it was listed as "Military", I assume) while Kukri Blades from Nepal fly through like drek through a goose. They've also had books held at the border for inspectors to investigate fully. It's gotten so insane that, at times, it was cheaper to drive down and pick them up from... Well, just south of the border. Raising petrol prices killed that possibility, however.
EDIT: This is why a lot of Canadians refuse to order *ANYTHING* online, even from Canadian sites. And doesn't even take into consideration US Sites that will flat-out tell you, "I'm sorry, sir, but we don't ship overseas.", no, I'm not joking.